Experiments with ' 5N labelled fertilizers often show that plants given fertilizer N take up more N from the soil than plants not given N-the priming effect or 'added nitrogen interaction' (ANI). This paper is a theoretical study of ANIs and how they can affect the interpretation of experiments with "N labelled fertilizers. ANIs can be 'real', if for example, fertilizer N increases the volume of soil explored by roots, or 'apparent', caused by pool substitution or by isotope displacement reactions. Pool substitution is the process by which added labelled N stands proxy for native unlabelled N that would otherwise have been removed from that pool. Microbial immobilization of N, whether driven by the decomposition of soil organic matter or by the decomposition of plant roots, can lead to pool substitution and is the dominant cause of apparent ANIs. Denitrification and plant uptake of N can also, under special circumstances, lead to pool substitution and thus give rise to apparent ANIs. Isotope displacement reactions, in which the added labelled N displaces native unlabelled N from a 'bound' pool, can lead to apparent ANIs but are only likely to be of significance in exceptional circumstances.The relationship between ANIs, 'A' values and N fertilizer uptake efficiencies are examined by means of a simple model for uptake of "N-labelled fertilizer by a cnp. A positive 'apparent' AN1 is accompanied by an 'A' value that changes as fertilizer applications increase. Likewise, a positive 'apparent' AN1 also causes fertilizer uptake efficiency to appear lower when measured by the uptake of I5N than when measured by the non-isotopic 'difference' method. I N T R O D U C T I O NNitrogen-15 labelled fertilizer is often used in experiments on the fate of fertilizer nitrogen, one of the aims being to distinguish between soil-derived (unlabelled) nitrogen and fertilizer-derived nitrogen. It is commonly (but not always, e.g. Leitch & Vaidyanathan, i983) observed in such experiments that plants given fertilizer N take up more unlabelled N from the soil than plants receiving no fertilizer N. Such an increase in N derived from soil following fertilizer additions is sometimes referred to as a 'priming' effect (Hauck & Bremner, 1976), a term introduced by Bingeman et al. (1953) in a paper about the effects of the addition of fresh organic material on the decomposition of organic matter already in the soil. The presence of plants is not essential for a priming effect; more unlabelled inorganic N is often observed in soil incubated with labelled inorganic N than in the corresponding soil incubated without labelled N. There has been considerable controversy over the cause and interpretation of this phenomenon (Broadbent

Background: Accelerating the clearance of therapeutic monoclonal antibodies (mAbs) from the

Most N testing methods require collecting, processing and analyzing of either soil or plant tissue samples. In 1987 we began a study to determine if chlorophyll readings of maize (Zea mays L.) made in the field with a portable chlorophyll meter could be used to predict soil N availabilities and sidedress fertilizer N requirements. The meter was tested over a 3‐yr period on 67 N response experiments located in central and southern Pennsylvania. At the six‐leaf stage of maize the chlorophyll meter reading of the fifth leaf was an accurate predictor of whether maize would respond to sidedress N fertilizer. A chlorophyll meter reading of 43.4 meter units was found to be the best critical level to separate responsive and nonresponsive sites. The chlorophyll meter method was similar in its accuracy in separating N responsive sites from nonresponsive sites to several proposed soil N availability tests including the soil NO3 concentration in the surface 20 cm of soil at planting, the UV absorbance at 200 nm of a 0.01 M NaHCO3 extract of the surface 20 cm of soil at planting, and the soil NO3 concentration in the surface 30 cm of soil 4 to 5 wk after plant emergence. The chlorophyll meter test was not well enough correlated with soil N supplying capability (r = 0.59) to determine sidedress N fertilizer rates for N responsive sites.

No abstract